CXCR4 is a G-protein coupled receptor whose natural endogenous ligand is the cytokine SDF-1 (stromal derived factor-1) or CXCL12. CXCR4 was first discovered as a co-receptor, with CD4, for the entry of T-cell line-tropic (X4) HIV-1 into T-cells. CXCR4 manipulation (in combination with granulocyte colony stimulating factor (G-CSF)) has proven to improve the outcome of haematopoietic (Broxmeyer et al., 2005) and endothelial progenitor cell (Pitchford et al., 2009) stem cell mobilization. The CXCR4-SDF-1 interaction is also a master regulator of cancer stem cell trafficking in the human body (Croker and Allan, 2008) and plays a key role in the progression and metastasis of various types of cancer cells in organs that highly express SDF-1 (Zlotnik, 2008). In view of these important biological functions mediated by CXCR4, small molecule antagonists of the CXCR4 receptor are promising as future therapeutics for stem cell transplantation and for the treatment of diseases such as diabetic retinopathy, cancer, HIV and AIDS.
Haematopoietic Stem Cell Mobilization
Haematopoietic stem cells or HSCs are widely used in the treatment of cancers of the haematopoietic system e.g. multiple myeloma and non-Hodgkin's lymphoma. Mobilisation and harvesting of HSCs allows the use of cytotoxic drugs to kill the tumour cells within the bone marrow. Subsequently the haematopoietic system can be re-introduced using the previously harvested stem cells.
Normally stem cells and progenitor cells are attracted to, and retained in, the bone marrow by the action of locally generated SDF-1 on CXCR4 expressed by such cells (see e.g. Lapidot et al., 2005). The stem cells used in transplantation can be mobilised from donors (allogeneic transplantation) or patients (autologous transplantation) after 4 or 5 days treatment with G-CSF. G-CSF is used in approximately 70 percent of the haematopoietic stem cell transplantations (the rest being bone marrow and umbilical cord transplants, Copelan, 2006) and acts by reducing the expression of CXCR4 in stem cells and reducing marrow SDF-1 levels (Levesque et al., 2003). Success in such treatments is based on the success of re-engraftment of the isolated stem cells. Should insufficient stem cells be isolated (<5×106/kg) then the re-engraftment is unlikely to be successful and the patient will not be treated. Multiple days of treatment with G-CSF may only result in sufficient yield in approximately 50% of cases. The CXCR4 antagonist Mozobil is now used to increase the efficiency of mobilisation, resulting in an approximate success rate of 90%. A single administration of CXCR4 antagonist, with a mobilisation delay of only a few hours, we predict will generate sufficient stem cells for effective transplantation (Devine et al., 2008). Such stem cells are we expect to show increased re-engraftment efficiency as they will not have had the expression of this receptor reduced by G-CSF treatment.
Non-Haematopoietic Stem and Progenitor Cell Mobilisation
An increase in plasma SDF-1 has been seen in a variety of models including heart infarction (Kucia et al., 2004; Wojakowski et al., 2004), stroke (Kucia et al., 2006), liver damage (Hatch et al., 2002), kidney damage, (Togel et al., 2005), pancreatic damage (Hess et al., 2003), bone fractures (Sata et al., 2005) and lung damage (Gomperts et al., 2006). It is hypothesised that the increase in SDF-1 is caused by the tissue damage and that gradients of this cytokine act to attract the relevant stem cells to the site of injury. This suggests a practical application of stem cell therapeutics for a wide range of injuries, all of which are regulated by the CXCR4/SDF-1 pathway. Interestingly Pitchford et al (2009) showed that different growth factors (VEGF, GCSF) administered with CXCR4 antagonists resulted in the mobilisation of different populations of stem cells, suggesting that appropriate combinations of factors could be used to isolate stem cells for the repair of specific tissues.
HIV and HIV Associated Pain
There is a known relationship between CCR5 and HIV (Alkhatib et al., 1996). The CXCR4 and CCR5 receptors act as entry cofactors for HIV infection. In brief, binding of the HIV gp120 protein to CD4 on the surface of CD4+ lymphocytes or macrophages exposes a domain in the gp120 protein which then also binds to the CCR5 or CXCR4 receptor, prior to viral insertion into the plasma membrane. CXCR4 antagonists have been shown to reduce the infectivity of X4 strains of the virus (Fransen et al., 2008), thus suggesting that the use of CXR4 antagonists would be effective treatments of HIV infection, especially in combination with CCR5 antagonists such as Maraviroc. The X4 strain of HIV is the most pathogenic, and these strains tend to predominate in the later stages of infection when neuropathic pain becomes an increasing problem for patients. Painful peripheral neuropathy affects approximately 50% of HIV patients. The HIV gp120 protein binds to CXCR4 and/or CCR5 which are expressed on neuronal and glial cells (Pardo et al., 2001; Oh et al., 2001), causing peripheral axonal damage (Melli et al., 2006) and initiating a cytokine-driven neurotoxic cascade involving glia and immune cells (Herzberg and Sagen, 2001; Milligan et al., 2000, 2001). The highly active anti-retroviral therapy (HAART) can also cause painful neuropathy (Dalakas et al., 2001), but this is predicted to become less common as improved therapies are used.
Therefore we expect CXCR4 antagonists to show both anti-viral, cognition enhancing and neuropathic pain relieving properties, and could be administered with other anti-(retro)viral therapies as well as analgesics such as amitryptiline, duloxetine and opiates.
Pain and Inflammation
Since the penetration of the blood brain barrier by leukocytes and monocytes is also influenced by the CXCR4 receptor, forms of brain inflammation and neurodegeneration (Bachis et al., J. Neuroscience, 2006, 26, 6771) whether virally induced or not, we expect to be amenable to therapy by CXCR4 antagonists (Kohler et al., 2008; McAndless et al., 2008). Similarly the expression of CXCR4 on primary sensory neurons suggests that antagonists of this receptor could act as analgesics in the control of pain (Oh et al., J. Neurosci. 2001 21, 5027-35). In addition the potent chemotactic action of SDF-1 on inflammatory cells (Gouwy et al., Eur J Immunol. 2011 41, 963-73), suggests that CXCR4 antagonists could serve as anti-inflammatory therapeutics.
Retinal Neo-Vascularisation
Retinal neo-vascularisation is a major cause of blindness in patients with diabetes and age related macular degeneration. The SDF-1/CXCR4 axis is strongly implicated in ocular neo-vascularisation and has been suggested as a target for treating diabetic retinopathy. Blockade of the CXCR4 receptor prevents the recruitment of endothelial progenitor cells, essential for the formation of the new microvessels which are part of the pathology of diabetic retinopathy (Lima e Silva et al., 2009). We expect the effects of CXCR4 antagonists to be additive with those of VEGF antibodies such as avastin. Thus we expect topical or intravitreal administration of CXCR4 antagonist to be an effective treatment of retinal degeneration with diabetes and ageing.
Cancer and Cancer Metastasis
Several types of cancers (including non-small cell lung, breast and neuroblastoma) express CXCR4, and SDF-1 is highly expressed in internal organs that represent the primary metastatic destinations of the corresponding cancer cells (for review see Ben Baruch, 2008). CXCR4 and SDF-1 are also implicated in the maintenance of cancer stem cells (Wang et al., 2006; Croker and Allan, 2008) and in the recurrence of tumours after radiation therapy. The role of the SDF-1/CXCR4 axis in cancer and cancer stem cells metastasis is discussed in Gelmini S et al., 2008. Blockade of CXCR4 prevented both the vasculogenesis and recurrence of glioblastoma multiforme tumours (Kioi et al., 2010), after radiation treatment. This ability to inhibit tumour derived vasculogenesis leads us to expect that CXCR4 antagonists will be effective when dosed with other anti-angiogenic agents including those that inhibit VEGF (e.g. avastin and aflibercept) and inhibitors of VEGF and PDGFreceptors such as cediranib, sunitinib, sorafenib, pazopanib, tivozanib vatalanib, vandertanib, brivanib, dovitinib, motesanib, telatinib and axitinib (Bhargava and Robinson Curr Oncol Rep (2011) 13:103-111). Other anti-angiogenic agents for use in conjunction with CXCR4 antagonists include those which inhibit EGF, angiopioetins, MMP-9, TNF, bFGF, CXCL8, HGF and TF (Nozawa et al., Proc Natl Acad Sci USA. 103, 12493-12498, 2006; Murdoch et al., Nature Reviews Cancer 8, 618-631 2008). Therefore we expect CXCR4 antagonists to be useful in the prevention of metastasis, and in anti-angiogenic treatment, as well as in cancer stem cell protection and maintenance, and in the sensitization of tumours to radiotherapy.